Preheat the oven to 400 degrees f. Spray a 9×13 inch baking dish with cooking spray. Spread refried beans evenly onto the bottom of the casserole dish.
Heat a large skillet to medium-high heat. Add the beef and onion and cook, stirring frequently and breaking up into chunks with a spatula until browned. Drain any excess grease. Stir in taco seasoning and tomatoes with juices from the can. Cook and stir until heated through.
Spoon the beef mixture on top of the beans. Sprinkle 1 cup of cheese over the beef. Top with frozen hash browns.
Place into the oven and bake for 35-40 minutes or until heated through and the potatoes are golden brown. Remove from the oven and top with the remaining cup of cheese. Return to the oven and bake for 3-4 minutes or until the cheese has melted.
Let sit 10 minutes before serving. Top with any toppings such as green onion, black olives, and sour cream.